FR24 (along with a few similar initiatives) is indeed out of the way for serious work. It is an amateur project, basing upon amateur-supplied information. Mind you, it never claims to be reliable or complete, as far as I observe. That said, I find it quite fascinating to follow, and so do my grandsons
And indeed, ADSB can never be the basis for any service as long as it is not universally mandatory. Even then, transponders can fail, like all equipment, electronics in particular.